Stimuli-Responsive Molecular Crystals: Structure-Property Relationships and Applications

Single-crystal-based smart materials offer significant advantages over their polymeric, polycrystalline, and amorphous counterparts owing to their long-range structural order, rapid response, and efficient energy transfer. Recent advances in mechanically compliant molecular crystals have made the flexible single-crystal materials attractive candidates for next-generation flexible devices. These materials respond to external stimuli such as mechanical force, temperature, light, electric and magnetic fields, chemical vapors, and so on, producing measurable outputs that are valuable for different applications. In particular, organic molecular crystals are lightweight, metal-free, readily tunable through well-established synthetic methodologies, and readily amenable to crystal engineering, enabling the rational design of materials with tailored properties. This review provides a comprehensive overview of stimuli-responsive organic molecular crystals, emphasizing their structure-property relationships, response mechanisms, and emerging applications. Different classes of mechanically, thermally, photo-, electrically, magnetically, and chemically responsive crystals are critically discussed together with the structural features governing their behavior. Particular attention is devoted to the mechanisms underlying crystal deformation, phase transitions, energy conversion, and stimuli-induced functional responses, as well as their applications in actuators, flexible electronics, optical waveguides, sensors, and other smart devices. Finally, the current challenges, emerging opportunities, and future design strategies for developing multifunctional organic crystalline materials with enhanced performance are highlighted.